Metal organic frameworks, also termed MOFs, form an interesting class of microporous or mesoporous substances having characteristics which allow their use in many technical fields, such as storage, separation or controlled release of chemical substances, catalysis and drug delivery. These potential applications are a direct consequence of the porous architecture of MOFs, which leads to high specific surface area of these materials.
MOFs are composed of two major components: a metal ion or cluster of metal ions and an organic molecule that plays the role of a ligand. The organic units are typically di-, tri- or polydentate ligands. The choice of metal and organic ligand has significant effects on the structure and properties of the MOF. For example, the metal coordination preference influences the size and shape of pores by dictating how many ligands can bind to the metal and in which orientation.
MOFs are produced almost exclusively by hydrothermal or solvothermal techniques, where crystals are slowly grown from a hot solution of metal precursor, such as metal salts, and bridging ligands. Since ligands in MOFs typically bind reversibly, the usual slow growth of crystals allows defects to be redissolved, resulting in a material with milli- and micrometer-scale crystals and a near-equilibrium defect density.
As a result of their industrial applicability, preparation of porous MOFs has attracted a interest over the last years, with numerous strategies having been described in the literature. For instance, WO2010058123 and EP1070 538 disclose methods for hydrothermal preparation of MOFs. Similarly, US20100076220 discloses a process for the obtaining of crystalline porous metal organic compounds in a liquid phase at elevated temperature.
However, the use of MOFs for industrial applications, such as storage materials for consumer products, demands an immense scale-up of their synthesis. Most of the available methodologies imply the use of high temperatures, high pressure and long crystallization times. The required conditions thus obstruct considerably the scale-up of MOF preparation which is required for industrial application.
A scaled-up process using mild conditions has been disclosed in US20090042000. This patent application discloses the preparation of kilogram quantities of Cu-benzene-1,3,5-tricarboxylic acid (1,3,5-BTC) by reacting an admixture of a copper nitrate with 1,3,5-BTC in the liquid phase at atmospheric pressure above 80° C. Still, long crystallization times between 15 and 96 hours are needed to obtain the crystalline MOF.
Shorter reaction times with mild conditions are disclosed in EP1373277. This document describes a method for the hydrothermal preparation of zinc terephthalate framework with good yields, the method comprising the reaction of an admixture of a zinc salt with terephthalic acid. The method requires the presence of a base (thriethylamine) and a lactam or a lactone solvent. However, even though the reaction time is shortened with respect to other methods, at least 3 hours of continuous stirring are needed to obtain a small quantity of MOF product, and no scale up has been attempted.
Additionally, all known preparation processes require the obtained MOF crystal to be separated from the mother liquor and further dried to obtain the final MOF product. The separation and drying steps add up to the already long preparation times.
Thus, even though a considerable improvement has been made in the art in the preparation of MOFs by hydrothermal methodologies, MOF preparation methods known in the art are not efficient for large-scale industrial preparation.